Wrap spring clutches are well known in a variety of forms and are used in a variety of applications. The basic operation of many wrap spring clutch designs involves utilizing a spring coil surrounding two shafts to transfer torque from one shaft to the other. Commonly the spring coil is fixed on one end to one of the shafts. When the clutch is activated, the unfixed end of the coil spring attaches to the other shaft and spins until the spring is wrapped down onto both shafts and torque is then transmitted from one shaft to the other. The clutch may be activated by various methods, including the use of an electromechanical switch.
It is known that the time between the activation of the clutch and the binding of the spring in these designs is dependent upon the distance between the inner surface of the spring coil and the shafts. Variations in the diameter of the spring coil can result in undesirable variations in the wrap spring clutch performance. In order to prevent these variations, it is known that the spring coil may be manufactured to tight tolerances. While manufacturing the spring coil to tight tolerances does improve the wrap spring clutch performance, it adds undesirable cost to the wrap spring clutch. It is also known that the inner diameter of the spring coil can be bored out after manufacture to minimize variations. This not only adds additional cost to the manufacture of the wrap spring clutch but it adds additional manufacturing steps as well. It would be highly desirable to be able to reduce the variation in wrap spring clutch performance without the costs associated with improved spring coil manufacturing.
Variations in spring coil diameter can lead to other manufacturing difficulties in wrap spring clutch production. Variations in spring coil diameter can result in variations in the position of the end tabs of the spring coil. Variation in end tab positions makes assembly of the wrap spring clutch difficult and costly. Variation in end tab position may be minimized by manufacturing the spring coil to tight tolerances. Tight tolerances, however, are undesirable since they add to the manufacturing cost of the wrap spring clutch. One known method of compensating for variations in end tab placement, is by manufacturing multiple attachment locations for the spring coil within the clutch assembly. This is a costly and undesirable solution. It would be desirable to have a design that reduced the variation in end tab placement such that design and assembly of the wrap spring clutch was simplified and costs were reduced.
The wrap spring clutch is a highly desirable design for use in many applications. The wrap spring clutch allows smooth engagement and has quick release properties that provide a valuable safety mechanism when used in applications such as automotive cruise control. A design is therefore needed that retains the positive characteristics of the wrap spring clutch while eliminating the deficiencies in manufacturing, cost, assembly, and performance associated with known designs.